The main indicator of success is the amount of fun you
had playing. If you do not solve the murder, but still have a great
time, then the night will obviously be a success.
It is often useful in the game to find your allies as soon as possible. Normally, to achieve your goals you need to get other people to help you. And that usually means negotiating with them, because they will most likely want something in return. Provided that their goals do not conflict with yours, there is no reason why you should not go for it.
Should you keep your goals secret? It depends on the goal. Some goals can easily be shared: "I have lost my watch - can you help me find it?" Goals that might reveal one of your terrible secrets are perhaps best left hidden.
If you need something from someone, and they ask you to pay for it, there is a good chance that the price will only rise as the game proceeds.
In Dinner and a Murder Mystery Games, you will have several goals and there are certificates given out for the following accomplishments:
Super Sleuth for those that solve the murder.
Wealthiest Player goes to the one with the most 'money' at the end of the party.
Best Costume goes to the most creative costume.
Best Performer goes to the person who plays their part the best.
The last two accomplishments will be voted on by every guest at the party.